Efficient synthesis and formulation of (R)‐(−)‐[11C]Deprenyl,a selective radioligand for the quantification of MAO‐B activity using PET

Carbon‐11 labeled (R)‐(?)‐Deprenyl is the tracer of reference for the quantification of monoamine oxidase (MAO)‐B activity with PET. In this paper, its radiosynthesis is re‐investigated and oriented towards the preparation of multi‐milliCuries of radiotracer. Typically, using no‐carrier‐added [¹¹C]methyl triflate as the alkylating agent, 140–190 mCi (5.1–7.0 GBq) of (R)‐(?)‐[¹¹C]Deprenyl was obtained within 30 min of radiosynthesis (including HPLC purification and formulation) with specific radioactivities ranging from 0.8 to 1.2 Ci/μmol (29.6–44.4 GBq/μmol). The high efficiency of these radiosyntheses allows for multi‐injection protocols and kinetic approaches for absolute quantification of the tracer. Copyright © 2002 John Wiley & Sons, Ltd.     

Radiosynthesis and in vivo evaluation of [11C]Ro‐647312: a novel NR1/2B subtype selective NMDA receptor radioligand

[2‐(3,4‐Dihydro‐1H‐isoquinolin‐2‐yl)‐pyridin‐4‐yl]‐dimethylamine, Ro‐647312 ( 1 ) represents a new novel class of NR1/2B subtype selective NMDA ligand. Ro‐647312 has been radiolabelled with carbon‐11 using [¹¹C]methyl triflate from the nor‐methyl compound 2 . The reaction was performed in acetone as solvent using aqueous NaOH as base. Following HPLC purification [¹¹C]Ro‐647312 ([¹¹C]‐ 1 ) was obtained in 6.9–9.2% (n = 3) radiochemical yield decay‐corrected based on starting [¹¹C]CO₂, with specific radioactivity measured at the end of the radiosynthesis ranging from 1.0 to 3.5 Ci/µmol (37–129 GBq/µmol). Radiochemical and chemical purities were assessed as >99 and >95%, respectively. Following i.v. injection of [¹¹C]‐ 1 in rat, the distribution of radioactivity was homogeneous in all brain structures and did not correlate with the known distribution of NR2B subunits. The radioactivity observed in plasma was also higher than any brain structure throughout the time course of the experiment. [¹¹C]‐ 1 does not possess the required properties for imaging NMDA receptors using positron emission tomography. Copyright © 2004 John Wiley & Sons, Ltd.     

Radiosynthesis of carbon‐11‐labelled GI181771, a new selective CCK‐A agonist

The novel CCK‐A agonist, (S)‐3‐(3‐{1‐[(isopropylphenylcarbamoyl)methyl]‐2,4‐dioxo‐5‐phenyl‐2,3,4,5‐tetrahydro‐1H‐benzo[b][1,4]diazepin‐3‐yl}ureido)benzoic acid, GI181771 ((S)‐ 1 ) has been isotopically labelled with carbon‐11 at its urea site using [¹¹C]phosgene in a one‐pot two‐step process, via the intermediate preparation of an [¹¹C]isocyanate derivative. Optimized conditions for the preparation of (S)‐[¹¹C]‐ 1 were the following: (1) Trapping of [¹¹C]phosgene (radiosynthesized from cyclotron‐produced [¹¹C]methane via [¹¹C]carbon tetrachloride using minor modifications of published processes) at room temperature for 1–2 min in 300 µl of acetonitrile containing 0.6 µmol of the appropriate (structurally complex) chiral‐amine giving the corresponding [¹¹C]isocyanate followed by (2) addition of an excess of 3‐aminobenzoic acid (40 µmol in 100 µl of THF) as the second amine giving the desired urea derivative (S)‐[¹¹C]‐ 1 and (3) high‐performance liquid chromatography (HPLC) purification on a semi‐preparative Waters Symmetry® C18. Starting from a typical 1.2 Ci (44.4 GBq) batch of [¹¹C]methane, 25–35 mCi (0.92–1.29 GBq, 6.8–9.6% decay‐corrected yield based on starting [¹¹C] methane, n = 5) of (S)‐[¹¹C]‐ 1 could be obtained within 35 min of radiosynthesis (HPLC purification and formulation as an i.v. injectable solution using a home‐made Sep‐pak®Plus C18 device included) with specific radioactivities ranging from 500 to 1500 mCi/µmol (18.5–55.5 GBq/µmol). The radiotracer preparation was a clear and colourless solution and its pH was between 5 and 7. As demonstrated by HPLC analysis, the radiolabelled product was found to be >99% chemically and radiochemically pure and the preparation was shown to be free of non‐radioactive precursors (starting amines) and radiochemically stable for at least 60 min. Finally, enantiomeric purity was found to be >99% according to chiral HPLC, demonstrating the absence of racemization during the process. Copyright © 2005 John Wiley & Sons, Ltd.     

Methylation of the thiophene ring using Carbon‐11‐labelled methyl iodide: formation of 3‐[11C]methylthiophene

This paper describes the radiosynthesis of 3‐[¹¹C]methylthiophene, chosen as a model reaction for the preparation of heteroaromatic methylthienyl compounds. Labelling was performed from the corresponding lithiothiophene derivative and [¹¹C]methyl iodide as the alkylating agent in THF at ?78°C. The conditions used were the following: (1) trapping for 2–3 min at ?78°C of the [¹¹C]methyl iodide in the THF solution containing the freshly prepared 3‐lithiothiophene; (2) Hydrolysis of the reaction mixture by adding 0.5 ml of the HPLC mobile phase and (3) HPLC purification. 3‐[¹¹C]Methylthiophene ([¹¹C]‐ 1 ) was collected in high yield as the unique peak of the HPLC radiochromatogram. Non‐reacted [¹¹C]methyl iodide was not present. Typically, 50–60 mCi (1.85–2.22 GBq) of 3‐[¹¹C]methylthiophene ([¹¹C]‐ 1 ) were obtained within 20 min of radiosynthesis (including HPLC purification) with specific radioactivities ranging from 0.6 to 1.0 Ci/μmol (22.2–37.0 GBq/μmol) starting from 180 to 200 mCi (6.66–7.40 GBq) of [¹¹C]CO₂ (10 μA, 10 min (6000 μC) irradiation). Copyright © 2002 John Wiley & Sons, Ltd.     

Radiosynthesis of [11C]SL25.1188 via [11C]CO2 fixation for imaging monoamine oxidase B

Carbon‐11‐labelled (S)‐5‐methoxymethyl‐3‐[6‐(4,4,4‐trifluorobutoxy)benzo[d]isoxazol‐3‐yl]oxazolidin‐2‐[¹¹C]‐one ([¹¹C]SL25.1188), a promising reversibly binding radiotracer for imaging central monoamine oxidase B, was rapidly prepared via an intramolecular cyclization reaction in an automated one‐pot procedure directly from [¹¹C]CO₂, thereby precluding the use of [¹¹C]COCl₂. Formulated [¹¹C]SL25.1188 was isolated in 12 ± 1% uncorrected radiochemical yield, based on starting [¹¹C]CO₂, with a specific activity of 37 ± 2 GBq/µmol at the end of synthesis (30 min; n = 3). Radiochemical and enantiomeric purities were both >99%. The methodology described herein offers an efficient production of [¹¹C]SL25.1188 at ambient temperature and is suitable for human imaging studies.     

Facile and improved synthesis of [11C]Me‐QNB

[¹¹C]Me‐QNB is a muscarinic acetylcholinergic receptor antagonist that has been used for the assessment of myocardial muscarinic receptors density in different cardiovascular pathologies. In the current technical note, we report a facile, highly efficient and fully automated method for the preparation of this radiotracer. The radiosynthesis was performed by reaction of [¹¹C]CH₃I with the desmethylated precursor (QNB) at room temperature using the captive solvent method. Excellent radiochemical yield (91.1 ± 2.4%, decay‐corrected) and radiochemical purity (>99.5%), and good specific activity (137 ± 5 GBq/µmol) were obtained when the purification was performed by reverse phase HPLC in overall synthesis time <31 min. Purification using solid‐phase extraction offered lower radiochemical yield (27.6 ± 3.1%, decay‐corrected) and radiochemical purity (>95%) but higher specific activity (244 ± 18 GBq/µmol) in shorter reaction times (<21 min). These results, especially concerning radiochemical yield, significantly improve those previously reported in which the reaction was performed in a vial and the purification step was based on ionic chromatography.     

Highlighting the versatility of the Tracerlab synthesis modules. Part 2: fully automated production of [11C]‐labeled radiopharmaceuticals using a Tracerlab FXC‐Pro

The field of radiochemistry is moving toward exclusive use of automated synthesis modules for production of clinical radiopharmaceutical doses. Such a move not only comes with many advantages but also presents radiochemists with the challenge of re‐configuring synthesis modules for production of radiopharmaceuticals that require non‐conventional radiochemistry while maintaining full automation. Herein, we continue our series of articles showcasing the versatility of the Tracerlab FX synthesis modules by presenting straightforward, fully automated methods for preparing a range of carbon‐11 labeled radiopharmaceuticals using a Tracerlab FX_C‐Pro. Strategies for production of [¹¹C]acetate, [¹¹C]carfentanil, [¹¹C]choline, [¹¹C]3‐amino‐4‐[2‐[(di(methyl)amino)methyl]phenyl]sulfanylbenzonitrile ([¹¹C]DASB), (+)‐a‐[¹¹C]dihydroterabenazine ([¹¹C]DTBZ), [¹¹C]flumazenil ([¹¹C]FMZ), meta‐hydroxyephedrine ([¹¹C]HED), [¹¹C]methionine, [¹¹C]PBR28, [¹¹C]Pittsburgh Compound B ([¹¹C]PiB), 1‐[¹¹C]methylpiperidin‐4‐yl propionate ([¹¹C]PMP), and [¹¹C]raclopride are presented. Copyright © 2011 John Wiley & Sons, Ltd.     

Radiosynthesis of [11C]paclitaxel

[¹¹C]paclitaxel, a potential solid tumor imaging agent, was synthesized by reacting [α‐¹¹C]benzoyl chloride with the primary amine precursor of paclitaxel. The time for synthesis, purification, and formulation was 38 min from end of bombardment with an average specific radioactivity of 49.9 GBq/μmol (1349 mCi/μmol) at end of synthesis. The average decay corrected radiochemical yield was 7% with greater than 99% radiochemical purity. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis of MMP inhibitor radiotracers [11C]methyl‐CGS 27023A and its analogs,new potential PET breast cancer imaging agents

[¹¹C]Methyl‐CGS 27023A ( 1a ) and its analogs [¹¹C]methyl‐2‐picolyl‐CGS 27023A ( 1b ), [¹¹C]methyl‐benzyl‐CGS 27023A ( 1c ), [¹¹C]methyl‐2‐nitro‐CGS 27023A ( 1d ), [¹¹C]methyl‐3‐nitro‐CGS 27023A ( 1e ), and [¹¹C]methyl‐4‐nitro‐CGS 27023A ( 1f ), novel radiolabeled matrix metalloproteinase (MMP) inhibitors, have been synthesized for evaluation as new potential positron emission tomography (PET) breast cancer imaging agents. The appropriate precursors for radiolabeling were obtained in four to five steps from starting material amino acid D ‐valine with moderate to excellent chemical yields. Precursors were labeled by [¹¹C]methyl triflate through ¹¹C–O‐methylation method at the aminohydroxyl position under basic conditions and isolated by solid‐phase extraction (SPE) purification to produce pure target compounds in 40–60% radiochemical yields (decay corrected to end of bombardment), in 20–25 min synthesis time. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis and PET evaluation of (R)‐[S‐methyl‐11C]thionisoxetine,a candidate radioligand for imaging brain norepinephrine transporters

Introduction: (R)‐3‐(2‐(methylthio)phenoxy)‐N‐methyl‐3‐phenylpropan‐1‐amine [(R)–thionisoxetine; 1 ] is a potent inhibitor of the norepinephrine transporter (NET). We aimed to label 1 with carbon‐11 (t_1/2 = 20.4 min) for evaluation as a radioligand for imaging NET in living brain with positron emission tomography (PET). Methods: Methyl 3‐(2‐((R)‐3‐(methylamino)‐1‐phenylpropoxy)phenylthio)‐propanoate (MPPP) and 1 were each prepared from o‐hydroxythiophenol in three steps. Treatment of MPPP with potassium t‐butoxide and [¹¹C]methyl iodide in tetrahydrofuran gave [S‐methyl‐¹¹C]thionisoxetine ([¹¹C] 1 ), which was purified with HPLC. The distribution of radioactivity in brain after intravenous injection of [¹¹C] 1 into cynomolgus monkey was followed with PET and the appearance of radiometabolites in plasma monitored with radio‐HPLC. Results: [¹¹C] 1 was obtained in high yield from [¹¹C]methyl iodide. Of the radioactivity injected into monkey, 2.4% entered brain. Ratios of radioactivity in thalamus, mesencephalon, occipital cortex and caudate to that in cerebellum at 93 min were 1.3, 1.2, 1.2 and 1.1, respectively. The radioactivity in plasma corresponding to unchanged radioligand decreased to 53% at 45 min, with the remainder represented by hydrophilic radiometabolites. Conclusions: MPPP is an effective precursor for ¹¹C‐methylation to [¹¹C] 1 , suggesting that the S‐γ‐propionic acid methyl ester protecting group may have wider value in the ¹¹C‐labeling of aryl methyl sulfides. However, the relatively low ratios of radioactivity to the cerebellum together with an unexpected accumulation of radioactivity in the caudate, makes [¹¹C] 1 an unpromising NET radioligand. Copyright © 2006 John Wiley & Sons, Ltd.     

An updated synthesis of [11C]carfentanil for positron emission tomography (PET) imaging of the μ‐opioid receptor

[¹¹C]Carfentanil ([¹¹C]CFN) is a selective radiotracer for in vivo positron emission tomography imaging studies of the μ‐opioid system that, in our laboratories, is synthesized by methylation of the corresponding carboxylate precursor with [¹¹C]MeOTf, and purified using a C2 solid‐phase extraction cartridge. Changes in the commercial availability of common C2 cartridges have necessitated future proofing the synthesis of [¹¹C]CFN to maintain reliable delivery of the radiotracer for clinical imaging studies. An updated synthesis of [¹¹C]CFN is reported that replaces a now obsolete purification cartridge with a new commercially available version and also substitutes the organic solvents used in traditional production methods with ethanol.     

Novel synthesis of [1‐11C]γ‐vinyl‐γ‐aminobutyric acid ([1–11C]GVG) for pharmacokinetic studies of addiction treatment

γ‐Vinyl‐γ‐aminobutyric acid (GVG, Vigabatrin^®), a suicide inhibitor of GABA‐transaminase (GABA‐T), has been suggested as a new drug for the treatment of substance abuse. In order to better understand its pharmacokinetics and potential side effects, we have developed a radiosynthesis of carbon‐11 (t_1/2=20 min) labeled GVG for positron emission tomographic (PET) studies. We report here a novel synthetic strategy to prepare the precursor and to efficiently label GVG with C‐11. 5‐Bromo‐3‐(carbobenzyloxy)amino‐1‐pentene was synthesized in five steps from homoserine lactone, including reduction and methylenation. This was used in a one‐pot, two‐step radiosynthesis. Displacement of bromide with no‐carrier‐added [¹¹C]cyanide followed by acid hydrolysis afforded [1‐¹¹C]GVG with decay corrected radiochemical yields of 27±9% (n=6, not optimized) with respect to [¹¹C]cyanide in a synthesis time of 45 min. Copyright © 2002 John Wiley & Sons, Ltd.     

Radiosynthesis of (S)‐5‐methoxymethyl‐3‐[6‐(4,4,4‐trifluorobutoxy)benzo[d]isoxazol‐3‐yl] oxazolidin‐2‐[11C]one ([11C]SL25.1188), a novel radioligand for imaging monoamine oxidase‐B with PET

Within a novel series of 2‐oxazolidinones developed in the past by Sanofi‐Synthélabo, SL25.1188 ((S)‐5‐methoxymethyl‐3‐[6‐(4,4,4‐trifluorobutoxy)benzo[d]isoxazol‐3‐yl]oxazolidin‐2‐one), a compound that inhibits selectively and competitively MAO‐B in human and rat brain (Ki values of 2.9 and 8.5 nM for MAO‐B, respectively, and ED_{50 (rat)}: 0.6 mg/kg p.o.), was considered an appropriate candidate for imaging this enzyme with positron emission tomography. SL25.1188 was labelled with carbon‐11 (T_1/2: 20.38 min) in one chemical step using the following process: (i) reaction of [¹¹C]phosgene with the corresponding ring‐opened precursor (1.2–2.5 mg) at 100°C for 2 min in dichloromethane (0.5 mL) followed by (ii) concentration to dryness of the reaction mixture and finally (iii) semi‐preparative HPLC purification on a Waters Symmetry^® C18. A total of 300–500 MBq of [¹¹C]SL25.1188 (>95% chemically and radiochemically pure) could be obtained within 30–32 min (Sep‐pak‐based formulation included) with specific radioactivities ranging from 50 to 70 GBq/µmol (3.5–7% decay‐corrected radiochemical yield, based on starting [¹¹C]CH₄). Copyright © 2008 John Wiley & Sons, Ltd.     

Carbon‐11 labelling of an N‐sulfonylamino acid derivative: a potential tracer for MMP‐2 and MMP‐9 imaging

Matrix metalloproteinases (MMPs) are a family of proteinases that play an important role in cancer. Non invasive imaging of MMPs would allow the evaluation of MMP activity in cancer and the assessment of the response of MMP inhibitor based therapies. In this paper, we investigated the synthesis and labelling by methylation with [¹¹C]CH₃I of an N‐sulfonylamino acid derivative, the (2R)‐3‐methyl‐2‐[[4‐[(4‐methoxybenzoyl)amino]benzenesulfonyl]amino] butanoic acid, a selective and high potent MMP‐2 and MMP‐9 inhibitor, for cancer imaging with positron emission tomography. Labelling of (2R)‐3‐methyl‐2‐[[4‐[(4‐hydroxybenzoyl)amino]benzenesulfonyl] amino] butanoic acid was carried out in a radiochemical yield of 50%–60% within 40 min with a specific activity of 11–26 GBq/µmol (EOS). In vitro inhibitory activity studies, biodistribution and in vivo serum stability in normal mice are also described. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis and characterization of novel radiofluorinated probes for positron emission tomography imaging of monoamine oxidase B

Monoamine oxidase B (MAO‐B), predominantly expressed in glial cells, plays an important role in neurotransmitter regulation, and MAO‐B activity relates to several neuronal diseases. Here, we aimed to develop a radiofluorinated MAO‐B imaging probe based on the structure of a selective MAO‐B inhibitor, MD‐230254. We synthesized and evaluated a series of compounds in vitro and in vivo. A series of fluorinated analogs of MD‐230254 were synthesized and evaluated for inhibitory potency and selectivity toward MAO‐B. 5‐[4‐(2‐[¹⁸F]Fluorobenzyloxy)phenyl]‐3‐(2‐cyanoethyl)‐1,3,4‐oxadiazol‐2(3H)‐one (2‐[¹⁸F]FBPO) was synthesized from a corresponding tributylstannyl precursor and [¹⁸F]CH₃COOF. Biodistribution after intravenous injection of 2‐[¹⁸F]FBPO was evaluated in male ddY mice with or without pretreatment by inhibitors. Among the compounds synthesized and evaluated, 2‐FBPO showed high inhibitory potency and selectivity toward MAO‐B comparable with MD‐230254. 2‐[¹⁸F]FBPO was successfully synthesized by an electrophilic reaction with a high radiochemical purity of more than 99%. 2‐[¹⁸F]FBPO was efficiently taken up by the brain and showed rapid blood clearance, which provided a brain/blood radioactivity ratio of 3.7 at 90 minutes postinjection. The brain radioactivity was significantly decreased by pretreatment with an MAO‐B selective inhibitor. The great potential of 2‐[¹⁸F]FBPO as an MAO‐B imaging probe, applicable to a variety of diseases, is indicated.     

Preparation of 4‐[11C]methylmetaraminol,a potential PET tracer for assessment of myocardial sympathetic innervation

The false adrenergic neurotransmitter [¹¹C]meta‐hydroxyephedrine ([¹¹C]HED) is currently the PET tracer of choice for assessment of myocardial sympathetic innervation. The molecule is metabolised in the 4‐position of the aromatic ring. The resulting radiolabelled metabolites need to be measured in order to obtain an arterial input function. Our aim was the development of a PET tracer with an increased metabolic stability relative to [¹¹C]HED. We selected 4‐methylmetaraminol as a candidate molecule for radiolabelling with ¹¹C (t_1/2 20.4 min). Our radiosynthetic approach towards 4‐[¹¹C]methylmetaraminol involved a palladium‐catalyzed cross‐coupling reaction of a protected 4‐trimethylstannyl derivative of metaraminol with [¹¹C]methyl iodide followed by removal of the protective groups. 4‐[¹¹C]methylmetaraminol was obtained in a final decay‐corrected radiochemical yield of 20–25% within a synthesis time of 60–80 min. The specific radioactivity at the end of the synthesis ranged from 18–37 to GBq/μmol. The unlabelled reference molecule, 4‐methylmetaraminol, was prepared in a 5‐step synthesis starting from metaraminol. A biological evaluation of 4‐[¹¹C]methylmetaraminol is in progress and the results will be reported elsewhere. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis and evaluation of [O‐methyl‐11C]4‐[3‐[4‐(2‐methoxyphenyl)‐ piperazin‐1‐yl]propoxy]‐4‐aza‐tricyclo[5.2.1.02,6]dec‐8‐ene‐3,5‐dione as a 5‐HT1A receptor agonist PET ligand

4‐[3‐[4‐(2‐Methoxyphenyl)piperazin‐1‐yl]propoxy]‐4‐aza‐tricyclo[5.2.1.02,6]dec‐8‐ene‐3,5‐dione (4), a potent and selective 5‐HT_1A agonist, was labeled by ¹¹C‐methylation of the corresponding desmethyl analogue 3 with ¹¹C‐methyl triflate. The precursor molecule 3 was synthesized from commercially available endo‐N‐hydroxy‐5‐norbornene‐2,3‐dicarboximide in two steps with an overall yield of 40%. Radiosynthesis of ¹¹C‐4 was achieved in 35 min in 20±5% yield (n=6) at the end of synthesis with a specific activity of 2600±250 Ci/mmol. In vivo positron emission tomography (PET) studies in baboon revealed rapid uptake of the tracer into the brain. However, lack of specific binding indicates that ¹¹C‐4 is not useful as a 5‐HT_1A agonist PET ligand for clinical studies. Copyright © 2008 John Wiley & Sons, Ltd.     

[carbonyl‐11C]Benzyl acetate: Automated radiosynthesis via Pd‐mediated [11C]carbon monoxide chemistry and PET measurement of brain uptake in monkey

[carbonyl‐¹¹C]Benzyl acetate ([¹¹C]1) has been proposed as a potential agent for imaging glial metabolism of acetate to glutamate and glutamine with positron emission tomography. [¹¹C]1 was synthesized from [¹¹C]carbon monoxide, iodomethane and benzyl alcohol via palladium‐mediated chemistry. The radiosynthesis was automated with a modified Synthia platform controlled with in‐house developed Labview software. Under production conditions, [¹¹C]1 was obtained in 10% (n=6) decay‐corrected radiochemical yield from [¹¹C]carbon monoxide in >96% radiochemical purity and with an average specific radioactivity of 2415 mCi/µmol. The total radiosynthesis time was about 45 min. Peak uptake of radioactivity in monkey brain (SUV=3.1) was relatively high and may be amenable to measuring uptake and metabolism of acetate in glial cells of the brain. Published in 2010 by John Wiley & Sons, Ltd.     

Synthesis and biological evaluation of S‐[11C]methylated mercaptoimidazole piperazinyl derivatives as potential radioligands for imaging 5‐HT1A receptors by positron emission tomography (PET)

The novel 2‐mercaptoimidazole derivatives, 1‐[4‐((2‐methoxyphenyl)‐1‐piperazinyl)butyl]‐2‐mercaptoimidazole ( 3 ) and methyl[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl] (2‐mercapto‐1‐methylimidazol‐5‐yl)methanamide ( 8 ), were efficiently labelled with ¹¹C through methylation of the thioketone function with [¹¹C]methyl iodide. The resulting radioligands 1‐[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl]‐2‐thio[¹¹C]methylimidazole ([¹¹C] 9 ) and methyl[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl] (2‐thio[¹¹C]methyl‐1‐methylimidazol‐5‐yl)‐methanamide ([¹¹C] 10 ) were synthesized in radiochemical yields of 20–30% (decay‐corrected, related to [¹¹C]CO₂) at a specific radioactivity of 0.2–0.4 Ci/µmol within 40–45 min including HPLC‐purification. The radiochemical purity exceeded 99%. The reference compounds 9 and 10 were tested in a competitive receptor binding assay to determine their affinity toward the 5‐HT_1A receptor. Both compounds exhibit excellent sub‐nanomolar affinities (IC₅₀=0.576±0.008 nM ( 9 ); IC₅₀=0.86±0.02 nM ( 10 )) for the 5‐HT_1A receptor while displaying a high selectivity towards the 5‐HT_2A subtype of receptors (IC₅₀>480 nM). By contrast, compound 9 also shows substantial binding for the alpha1‐adrenergic receptor (IC₅₀=3.00±0.02 nM) when compared with compound 10 (IC₅₀=54.5±0.6 nM). Preliminary biodistribution studies in rats showed an initial brain uptake of 1.14±0.11 and 0.37±0.04% ID/g after 5 min, which decreased to 0.18±0.04 and 0.16±0.01% ID/g after 60 min for compounds [¹¹C] 9 and [¹¹C] 10 , respectively. For both compounds, the cerebellum and rest of the brain uptake are very similar at the different time points. Unlike [¹¹C] 9 , the radioligand [¹¹C] 10 has significant uptake and retention in the adrenal glands. Due to their washout from the brain compounds [¹¹C] 9 and [¹¹C] 10 seem not to be good candidates as radioligands for imaging 5‐HT_1A receptors by PET. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of a 11C‐labelled taxane derivative by [1‐11C]acetylation

The ¹¹C‐labelling of the taxane derivative BAY 59‐8862 ( 1 ), a potent anticancer drug, was carried out as a module‐assisted automated multi‐step synthesis procedure. The radiotracer [¹¹C]1 was synthesized by reacting [1‐¹¹C]acetyl chloride ( 6 ) with the lithium salt of the secondary hydroxy group of precursor 3 followed by deprotection. After HPLC purification of the final product [¹¹C]1 , its solid‐phase extraction, formulation and sterile filtration, the decay‐corrected radiochemical yield of [¹¹C]1 was in the range between 12 and 23% (related to [¹¹C]CO₂; n=10). The total synthesis time was about 54 min after EOB. The radiochemical purity of [¹¹C]1 was greater than 96% and the chemical purity exceeded 80%. The specific radioactivity was 16.8±4.7 GBq/µmol (n=10) at EOS starting from 80 GBq of [¹¹C]CO₂. Copyright © 2006 John Wiley & Sons, Ltd.